Due to its high oxidation potential, bactericidal properties, ease of on-site generation from air or oxygen, and ease of destruction to form ordinary oxygen, ozone has become widely used in a large number of applications such as water treatment, food processing, odor removal, aquaculture and soil and ground water remediation. In most applications ozone, produced in an electrical discharge of air or oxygen, is dissolved in water, usually under high pressure. As examples, the water treated can be municipal drinking water, bottled water, water to be used in beverages, wastewater, water used in aquaculture and contaminated ground water. Ozonated water also is widely used as a bactericidal rinse for meats and vegetables being processed for food and as a cleaning agent for semiconductor chips and other electronic parts. In these and many other applications, it is important to know the concentration of dissolved ozone throughout the process. The ozone concentration integrated over time represents a dose to the water being treated, the change in ozone concentration over time is often indicative of the original concentrations of contaminants in the water, and the residual concentration is important for knowing when the treatment process is complete and the water can be safely used. Thus, there are many applications requiring the measurement of dissolved ozone in water. In many cases the water being treated with ozone is not of high purity and may contain dissolved organic, inorganic and biological compounds and particulate matter. The presence of such materials makes measurement of dissolved ozone challenging in all but high purity water. In some applications, such as the semiconductor industry, ozone may be dissolved into water containing strong acids and other oxidants such as hydrogen peroxide.
Ozone in impure or “dirty” water has been measured by both batch and automated methods. Methods for measuring dissolved ozone were reviewed recently by Majewski (2012). A colorimetric method based on bleaching of the indigo dye has long been used in a batch mode (e.g., Bader and Hoigne, 1981) and is the basis of Standard Method 4500-O3 (Standard Methods Committee, 1997). Although considered accurate, this method is cumbersome, requiring hand mixing of individual water samples with a reagent solution followed by measurement of the decrease in absorbance at ˜600 nm using a colorimeter. The method is often used as a calibration method for automated instruments.
Absorption of UV light has long been used as an automated method for measurements of gas-phase ozone (e.g., Bognar and Birks, 1996; Wilson and Birks, 2006) and of ozone dissolved in pure solvents with high precision and accuracy. The ozone molecule has an absorption maximum at 254 nm, coincident with the principal emission wavelength of a low-pressure mercury lamp. There are commercial instruments available for the direct measurement of ozone in high purity water and other solvents, but those instruments cannot be used for measuring ozone in drinking water and other “dirty” water because of the presence of UV-absorbing compounds and/or particles that both absorb and scatter UV radiation. In addition, the concentrations of those interfering species often change upon exposure to ozone due to chemical reactions, further complicating direct UV absorbance measurements of dissolved ozone.
At present, dissolved ozone is most commonly measured using sensors separated from the sample water by an ozone-selective membrane. The membrane allows ozone to diffuse to the sensor while minimizing the diffusion of interfering compounds. Two types of sensors are typically used in membrane-based ozone monitors, amperometric and polarographic sensors. In amperometric sensors, ozone participates in an electrochemical reaction, and the resulting electrical current is measured. These sensors have the disadvantage of producing a response to other oxidizers in the sample as well. Polarographic sensors, which vary the potential applied to an electrode, are more selective for ozone, because different species are detected as the applied potential is scanned. Although widely used, the membranes of such sensors are easily fouled and require routine maintenance.
Gas phase sensors and detectors also are used that employ a gas stripping or sparging process instead of a membrane. A stream of inert gas or ozone-free air is bubbled through the sample, or a liquid sample is sprayed into a stream of air or inert gas, and a fraction of the dissolved ozone is transferred to the gas-phase in a way that establishes an approximate equilibrium between dissolved and gas-phase ozone. The ozone concentration is then measured in the gas phase using a UV photometer or other device such as a heated metal oxide semiconductor (HMOS) ozone sensor. In theory, the gas-phase concentration is related back to the dissolved ozone concentration using Henry's Law, which states that the dissolved concentration of ozone (c) is proportional to the partial pressure (p) of ozone in the air over the surface of the water (p),c=p/H  (1)where H is the Henry's constant. In order to convert ozone concentration in the gas phase to partial pressure the values of temperature of pressure of the gas also are required. Ozone has a low solubility in water and thus a high Henry's Law constant. However, Henry's Law only applies if equilibrium is established between the gas and liquid phases. Since the gas stripping process does not always achieve a perfect equilibrium, the assumption of a Henry's Law relationship can cause a significant error in the ozone measurement. The commercially available instruments based on this approach use continuous flows of both water and sparging gas, and the instruments are calibrated using the indigo blue or other method to correct for deviations from the Henry's Law equilibrium. However, the Henry's Law constant is strongly affected by the ionic strength, temperature and pH of the water, which adds additional uncertainty to the ozone measurement, especially in impure water.
Sparging (bubbling or stripping) of ozone from solution followed by measurement of ozone in the gas phase has the advantage of measuring the dissolved ozone in the absence of UV-absorbing interferences that remain in the water. However, instruments designed around this principle in the past have incorporated very large and cumbersome sparging chambers and rely on a fixed value of the Henry's Law constant that partitions ozone between the liquid and gas phases. An important way in which the invention described here differs from previously known methods for measuring dissolved ozone based on sparging is that it does not rely on establishment of an equilibrium between ozone in the gas and liquid phases and thus does not depend on the value of the Henry's Law constant, which varies with ionic strength, temperature and pH.